If no epacems_api_key, return None instead of throwing an exception

[e-belfer]

Overview

What problem does this address?
Right now if there's no EPACEMS_API_KEY set in the environment, an error is thrown whether or not a user is actually archiving EPA CEMS data. This came up in another repository importing the pudl-archiver code to run an archiver.

What did you change in this PR?
Switched from using os.environ['EPACEMS_API_KEY'] which raises an error if this doesn't exist, to os.environ.get("EPACEMS_API_KEY") which will return None. This will still fail if the EPA CEMS API key is being called in the archiver, but will allow people to run other archivers without making a fake EPA CEMS API key environment variable.

Testing

How did you make sure this worked? How can a reviewer verify this?

To-do list

Tasks

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1. Update relevant documentation - like comments, docstrings, README, release notes, etc.
2. Review the PR yourself and call out any questions or issues you have

[zschira]

Looks good!